The present invention relates to image forming apparatuses that can carry out accurate exposure by correcting various types of shifts that could not be corrected by merely correcting the physical position shifts of a plurality of light sources.
In recent years, since higher speed and higher density of writing are expected of image forming apparatuses, a plurality of lines are being written simultaneously using multiple beams.
In this case, a laser diode (LD) array 140AL in which a plurality of light emitting sections are formed integrally as is shown in FIG. 2a is used as an exposure section 140. Further, this exposure section is sometimes not placed vertical to the main scanning direction (placement along the auxiliary scanning direction: FIG. 2a), but is used by placing it in an inclined manner with respect to the auxiliary scanning direction as is shown in FIG. 2b. 
Because of this inclined placement of the exposure section, the intervals in the auxiliary scanning direction of the plurality of lines formed along the main scanning direction become narrower, and high density is realized in the auxiliary scanning direction.
In this case, since position shift occurs physically in the main scanning direction as is shown in FIG. 2b, it is necessary, as is shown in FIG. 3, to carry out writing after adjusting the position of starting the writing for each beam, and correcting the position shift due to inclined placement.
As a method of correcting shift in inclined placement of such LD array 140AL, it is possible to measure the spacing between neighboring LDs in the main scanning direction by allowing the LD which is the target of measurement to emit light and measuring the distance of the shift in the main scanning direction using a photo sensor. Further, by reflecting the amount of shift obtained from the measurement result in the timing of generation of the main scanning direction reference signal (INDEX) or in the phase adjustment of the pixel clock, aligning of the starting position of writing of multiple beams can be carried out.
Further, regarding the shift correction of multiple beams when the LD array is placed in an oblique manner, a proposal has been made in the following Unexamined Japanese Patent Application Publication No. 2006-305874.
In the prior art of the above Unexamined Japanese Patent Application Publication No. 2006-305874, the shift in the main scanning direction of a plurality of LDs (two LDs among these) is detected and corrected electrically. Here, the measurement target is the SIND signal which is made from the INDEX signal generated by receiving the light of the LD beam by the INDEX sensor and is made by synchronizing the INDEX signal with the pixel clock.
Further, the reference LD used for light reception of the INDEX sensor is changed, and SIND signals are generated such as SIND1 for a certain line and SIND2 for the next line. In addition, the interval from SIND1 to SIND2 (the length of approximately one line) is measured by a counter. Because of this, the correction value obtained by measurement is reflected in the writing CLK correcting section (delay line) of each of the LDs, the INDEX generation timing, or in the H-Valid generating section.
However, in the above correction method, the inventors of the present invention found out that, in spite of carrying out correction of the starting position of writing by executing adjustment by carrying out pixel clock phase adjustment using a delay device and a PLL or by changing the timing of generating the INDEX signal, fluctuations are generated in the starting position of writing.
This is because, even if the position shift is detected by measuring the LD position using a photo sensor, this is merely grasping the amount of position shift in the physical position of the LD and carrying out correction based on that amount of position shift.
In addition, the range for setting in which the actual LD position shift correction is reflected is within the range of the processing circuit that carries out adjustment of the pixel clock phase adjusting section, and the pixel clock output after the phase adjustment has been carried out is outside the range of correction.
When fluctuations in the electrical characteristics (electrical shifts) occur in the delay devices or PLL incorporated in the pixel clock phase adjusting section, or when shifts occur relative to the assumed values in the power supply voltage or temperature of circuit elements, since there is no means for detecting the amount of shift newly caused by those parts, variations remain in the actual starting position of writing, and pixel shifts occur in the main scanning direction.
Further, due to the oblique placement of LD arrays for higher densities as explained above, there is a big problem that the image quality of the image forming apparatus decreases.
Further, at the present point in time, in apparatuses such as image forming apparatuses using oblique placement of LD arrays, technologies made by considering the above problem, or effective methods for solving the above problem did not exist.
The present invention solves the above problems, and an object of the present invention is to realize an image forming apparatus in which it is possible to generate accurate signals in a condition in which the residual shift components are eliminated that are generated due to variations in various electrical characteristics present in signal generating sections or the like, that generate synchronized signals according to the physical shifts of a plurality of light sources.